High manganese steel sheet having excellent damping property, and manufacturing method therefor

ABSTRACT

Provided is a high Mn steel sheet and a manufacturing method therefor, the steel sheet comprising, by wt %: 0-0.1% or less of C; 8-30% of Mn; 0.1% or less of P; 0.02% or less of S; 0.10 or less of N; 0-1.00 of Ti; and Fe and inevitable impurities, wherein a microstructure has epsilon martensite and austenite, and the average particle diameter of the martensite and the austenite is 2 μm or less.

TECHNICAL FIELD

The present disclosure relates to a high Mn steel sheet having excellentdamping properties, provided as a steel sheet for a vehicle orconstruction equipment and used in a location requiring dampingproperties for noise reduction.

BACKGROUND ART

In recent years, noise reduction in vehicle manufacturing and buildingmaterials is a problem which manufacturers have needed to solve. In thecase of vehicle manufacturers, it is particularly necessary to havedamping properties in addition to excellent mechanical properties incomponents such as engine parts, oil pans, and the like, in which asignificant noise is generated. In the case of building materials,recently, due to the strengthening of regulations relating to noisebetween floors, the development of a damping steel sheet as a floorplate of a duplex building including an apartment building is required.

High Mn damping steel is steel having good damping properties andexcellent mechanical properties, since noise energy may be converted tothermal energy due to interface sliding of epsilon martensite when anexternal impact is applied.

DISCLOSURE Technical Problem

An aspect of the present disclosure may provide a high Mn steel sheethaving excellent damping properties and a method for manufacturing thesame.

Technical Solution

According to an aspect of the present disclosure, a high Mn steel sheet,having excellent damping properties, includes: 0 wt % to 0.1 wt % ofcarbon (C), 8 wt % to 30 wt % of manganese (Mn), 0.1 wt % or less ofphosphorus (P), 0.02 wt % or less of sulfur (S), 0.1 wt % or less ofnitrogen (N), 0 wt % to 1.0 wt % of titanium (Ti), and iron (Fe) andinevitable impurities, wherein a microstructure contains epsilonmartensite and austenite, and an average particle diameter of each ofthe martensite and the austenite is 2 μm or less.

According to another aspect of the present disclosure, a method formanufacturing a high Mn steel sheet, having excellent dampingproperties, includes: heating a steel sheet, satisfying the compositionrange described above, to a heat treatment temperature of Ac1° C. toAc3+50° C. at a heating rate of 0.01° C./s to 200° C./s; maintaining thesteel sheet for 0.01 second to 24 hours at the heat treatmenttemperature; and cooling the steel sheet to room temperature at acooling rate of 0.01° C./s or more.

Advantageous Effects

According to an exemplary embodiment in the present disclosure, a highMn steel sheet having excellent damping properties is provided, and isused for structural components for a vehicle and building materials suchas a flooring material, in which a noise reduction is required.

DESCRIPTION OF DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing (s) will be provided by the Office upon request andpayment of the necessary fee.

FIG. 1 is a view illustrating the microstructures of the Example heattreated at 600° C. and the Comparative Example heat treated at 700° C.to 1000° C.

FIG. 2 is a drawing illustrating a dilatometer cycle of the heattreatment illustrated in FIG. 1.

FIG. 3 is a graph illustrating the results of specific damping capacity(SDC) measured by the internal resistance method with respect to theExample and the Comparative Example (4).

BEST MODE FOR INVENTION

Hereinafter, the present disclosure will be described in detail.

The present disclosure relates to a high Mn steel sheet having excellentdamping properties and a method for manufacturing the same. The high Mnsteel sheet includes 0 wt % to 0.1 wt % or less of carbon (C), 8 wt % to30 wt % of manganese (Mn), 0.1 wt % or less (including 0%) of phosphorus(P), 0.02 wt % or less (including 0%) of sulfur (S), 0.1 wt % or less(including 0%) of nitrogen (N), 0 wt % to 1.0 wt % (excluding 0%) oftitanium (Ti), and iron (Fe) and inevitable impurities. A microstructureof the steel sheet has epsilon martensite and austenite, and an averageparticle diameter of each of the martensite and the austenite is 2 μm orless.

A specific steel composition of the steel sheet according to the presentdisclosure and the reasons for restricting the steel composition thereofare as follows.

If the addition amount of carbon (C) exceeds 0.1%, carbides may beexcessively precipitated, and the hot workability and elongation arelowered, and the damping capability may be significantly reduced. Thus,the addition amount of C is limited to 0.1% or less.

Manganese (Mn) is an element essential for stably securing an austenitestructure and is an element for increasing the stacking fault energy. Ifan addition amount of Mn is less than 8%, martensite, degradingformability, is formed. Thus, strength increases but ductilitysignificantly decreases. Moreover, the stacking fault energy is lowered,so austenite, partially formed, may be easily transformed into epsilonmartensite. Thus, a lower limit of Mn is limited to 8%. On the otherhand, if the addition amount of Mn exceeds 30%, manufacturing costs maybe increased due to a large amount of Mn, and slab cracking may becaused by an increase in an amount of phosphorus (P) in steel. Moreover,as the addition amount of Mn increases, internal grain boundaryoxidation significantly occurs when a slab is reheated. Thus, an oxidedefect on a surface of a steel sheet is caused. Moreover, surfaceproperties are degraded in hot dip galvanizing. Thus, an upper limit ofthe addition amount of Mn is limited to 30%.

Phosphorus (P) and sulfur (S) are inevitably included when steel isproduced. The content of P is preferably limited to 0.1% or less(including 0%), and the content of S is preferably limited to 0.02% orless (including 0%). In detail, in the case of P, segregation occurs, sothe workability of steel may be reduced. In the case of S, coarsemanganese sulfide (MnS) is formed, so a defect such as flange crackingmay be generated, and the hole expansion of a steel sheet may bereduced. Thus, an addition amount thereof should be significantlysuppressed as much as possible.

Nitrogen (N) is an element, inevitably contained during the production,so an addition range of N is preferably limited to 0.1% or less(including 0%).

Titanium (Ti) is a strong carbide forming element, combined with carbonto form a carbide. The carbide formed at this time is an elementeffective in refining a grain size by inhibiting crystal grain growth.Moreover, when Ti is added in combination with boron, a high temperaturecompound is formed in a columnar-grained boundary to prevent grainboundary cracking from occurring. In addition, Ti has a scavengingeffect in which a fraction of each of C and N is lowered by forming acompound with C and N. Thus, Ti is an essential element for improvingdamping capabilities. However, if an addition amount of Ti exceeds1.00%, an excessive amount of Ti may be segregated in a crystal grainboundary and grain boundary embrittlement may be caused, or aprecipitation phase may be excessively coarse. Thus, a grain growtheffect may be lowered. Therefore, the addition amount of Ti is limitedto being 1.0% or less.

The high Mn steel sheet according to another aspect to the presentdisclosure additionally includes one, two, or more among 0 wt % to 3 wt% of silicon (Si), 0.005 wt % to 5.0 wt % of chromium (Cr), 0.005 wt %to 2.0 wt % of nickel (Ni), 0.005 wt % to 0.5 wt % of niobium (Nb),0.0001 wt % to 0.01 wt % of boron (B), 0.005 wt % to 0.5 wt % ofvanadium (V), and 0.005 wt % to 1 wt % of tungsten (W).

Silicon (Si) is a solid solution strengthening element, and is anelement for increasing yield strength by reducing a grain size by asolid solution effect. Thus, Si is necessary to secure strength.However, if an addition amount of Si is increased, during hot rolling,silicon oxide is formed on a surface of a steel sheet. Thus, picklingproperties are deteriorated, so surface qualities of a steel sheet maybe deteriorated, as a disadvantage. Moreover, due to addition of a largeamount of Si, the weldability of steel may be significantly lowered.Thus, an upper limit of the addition amount of Si may be limited to 3%.

Chromium (Cr) reacts with external oxygen during a hot rolling orannealing operation, and a Cr-based oxide film (Cr₂O₃) having athickness of 20 μm to 50 μm is preferentially formed on a surface of asteel sheet, so Mn and Si, contained in the steel, are prevented frombeing eluted into a surface layer. Thus, Cr may act as an element forcontributing to stabilization of a structure of a surface layer andimproving plating surface properties.

However, if Cr is included in an amount less than 0.005%, the effectdescribed above is insignificant. If Cr is included in an amountexceeding 5.0%, chromium carbide is formed, so workability and delayedfracture properties are deteriorated. Thus, an upper limit of thecontent of Cr may be limited to 5.0%.

Nickel (Ni) contributes to the stabilization of the austenite, which isnot only advantageous for improving the elongation but also contributingeffectively to the high temperature ductility. If the content of Ni, astrong high temperature toughness improving element, is less than0.005%, an effect on high temperature toughness is insignificant. If anadded content of Ni increases, Ni has a significant effect on preventionof delayed fracture and slab cracking. However, material costs may behigh, so production costs may be increased. Thus, the content of Ni maybe limited to 0.005% to 2.00.

Niobium (Nb) is a carbide forming element for forming carbide by beingcombined with carbon in the steel. In the present disclosure, Nb may beadded for the purpose of increasing the strength and refining the grainsize. According to the related art, Nb forms a precipitate at a lowertemperature as compared to Ti, so Nb is an element having significanteffects of refining a crystal grain size and strengthening precipitationby formation of a precipitate. Thus, Nb may be added in an amount of0.005% to 0.5%. However, if an addition amount of Nb is less than0.005%, the effect described above is insignificant. On the other hand,if Nb is added in an amount exceeding 0.5%, an excessive amount of Nb issegregated in a grain boundary, thereby causing grain boundaryembrittlement, or a precipitate may be excessively coarse, therebylowering a growth effect of a crystal grain. Moreover, in a hot rollingprocess, recrystallization is delayed, so rolling load may be increased.Thus, the addition amount of Nb may be limited to 0.005% to 0.5%.

Vanadium (V) and tungsten (W) are elements for forming carbonitride bybeing combined with C and N together with Ti. In the present disclosure,since a fine precipitate is formed at a low temperature, a precipitationstrengthening effect may be obtained. Moreover, V and W may be importantelements for securing austenite. However, if two elements are added insmall amounts, less than 0.005%, the effect described above isinsignificant. On the other hand, if V is added in an amount exceeding0.5% and W is added in an amount exceeding 1.0%, a precipitate isexcessively coarse, so a grain growth effect may be lowered, which maycause hot brittleness. Thus, the addition amount of V may be limited to0.005% to 0.5%, while the addition amount of W may be limited to 0.005%to 1%.

Boron (B) is added with Ti, and thus a high-temperature compound isformed in a grain boundary, so grain boundary cracking may be prevented.However, if B is added in a small amount of 0.0001% or less, there maybe no effect. If B is added in an amount of 0.01%, a boron compound isformed, so surface properties may be deteriorated. Thus, a range of Bmay be limited to 0.0001% to 0.01%.

Hereinafter, a method for manufacturing a high Mn steel sheet havingexcellent damping properties will be described.

In the method for manufacturing a high Mn steel sheet according to thepresent disclosure, steel, which has the composition and compositionrange, and in which a microstructure is comprised of a austenite as mainphase, is heated at a heating rate of 0.01° C./s to 200° C./s, andmaintained at a heat treatment temperature of Ac1° C. to Ac3+50° C. for0.01 second to 24 hours, and is then cooled to room temperature at acooling rate of 0.01° C./s or more.

The high Mn steel sheet may be a cold rolled steel sheet or a hot rolledsteel sheet.

A microstructure of the high Mn steel sheet has epsilon martensite andaustenite.

In the heating, if a heating rate exceeds 200° C./s, Ac1 and Ac3temperatures are raised. Thus, even when heat treatment is performed inthe range of the present disclosure, there is a problem in which anaverage particle diameter of a microstructure exceeds 2 μm. Therefore,an upper limit of a heating rate is limited to 200° C./s. If the heatingrate is 0.01° C./s or less, during an operation, coarse carbides may begenerated due to phase instability. Therefore, the heat treatment shouldbe performed at a rate of 0.01° C./s or more.

If the heat treatment is performed at a temperature less than Ac1,transformation does not proceed, so there may be a problem in which aheat treatment effect cannot be obtained. If a heat treatmenttemperature exceeds Ac3+50° C., there may be a problem in which anaverage particle diameter of a microstructure exceeds 2 μm. Thus, theheat treatment temperature is limited to Ac1° C. to Ac3+50° C.

If the heat treatment time is less than 0.01 second, the application ofrecrystallization and recovery is insignificant, so an effect of heattreatment maybe not obtained. If the heat treatment time exceeds 24hours, there may be a problem in a process in which excessive oxidationoccurs, so base iron is removed, and excessive heat treatment costs areconsumed, as well as a problem in a microstructure in which amicrostructure significantly grows, so an average particle diameterthereof is greater than a target particle diameter.

If the cooling is performed at a cooling rate less than 0.01° C./s,there may be problems in which an average particle diameter of amicrostructure is increased during the cooling, and coarse carbides aregenerated due to phase instability. Thus, a lower limit of a coolingrate is limited to 0.01° C./s. There is no upper limit of a coolingrate. As the cooling rate is increased, it may be more advantageous tosecure epsilon martensite and secure a fine average particle size.

MODE FOR INVENTION

Hereinafter, an Example of the present disclosure will be described indetail. The following example is for the purpose of understanding thepresent disclosure and is not intended to limit the present disclosure.

A cold rolled steel sheet, including 0.02 wt % of C, 17 wt % of Mn, 0.01wt % of N, 0.008 wt % of P, 0.008 wt % of S, 0.03 wt % of Ti, and Fe andinevitable impurities, was heated at a heating rate of 5° C./s, and wasthen maintained for the heat treatment time at a heat treatmenttemperature illustrated in Table 1. Then, the heat treated steel sheetwas cooled to room temperature at a cooling rate of 5° C./s.

Regarding the steel sheet, having been heat-treated and cooled, anaverage particle diameter of a microstructure and the fraction ofepsilon martensite were investigated, and a result thereof isillustrated in Table 1 and FIG. 1.

TABLE 1 Heat Heat Area Fraction Treatment Treatment Particle (%) ofTemperature Time Diameter Epsilon Classification (° C.) (min) (μm)Martensite Example 600 30 1.23 6.2 Comparative 700 30 2.3 3 Example 1Comparative 800 30 3.6 14.9 Example 2 Comparative 900 10 6.7 16.8Example 3 Comparative 1000 30 6.7 34.6 Example 4

As disclosed in Table 1 and FIG. 1, the Example, heat treated at 600°C., is compared with the Comparative Examples (1 to 4), heat treated at700° C. to 1000° C. In Comparative Example (1), in which a heattreatment temperature was 700° C., the area fraction of epsilonmartensite was lower, and a particle diameter was greater, as comparedto the Example, heat treated at 600° C.

Moreover, in the case of Comparative Examples (2 to 4) in which heattreatment temperatures were 800° C., 900° C., and 1000° C.,respectively, the area fraction of epsilon martensite was greater, ascompared to the Example, heat treated at 600° C. However, a particlediameter of the Example, heat treated at 600° C., was smaller than aparticle diameter of the Comparative Examples (2 to 4), heat treated at700° C. to 1000° C.

Meanwhile, in the case of Example, heat treated at 600° C. according tothe present disclosure, an average particle diameter of a microstructurewas 2 μm or less.

FIG. 2 is a drawing illustrating a dilatometer cycle of heat treatmentillustrated in FIG. 1.

In FIG. 2, Ac1 and Ac3 are confirmed, and the Example is a result inwhich a heat treatment was performed at Ac3 +30° C.

In FIG. 3, the Example, heat treated at 600° C., and Comparative Example(4), heat treated at 1000° C., were measured using a frictioncoefficient method. A result of measuring Specific Damping Capacity(SDC) is illustrated therein.

Here, SDC indicates the damping capacity (a property in which an objectabsorbs vibrations).

Referring to FIGS. 1 and 3, a room temperature SDC value of dampingsteel having a microstructure according to the Example, heat treated at600° C., is a value 2.5 times that of damping steel according toComparative Example (4). In other words, a SDC value of Example, heattreated at 600° C., is 0.00025, and a SDC value of Comparative Example(4), heat treated at 1000° C., was measured as 0.0001.

The area fraction of epsilon martensite of the Example, heat treated at600° C., was relatively low. However, a particle diameter thereof wassmall, so a structure may be finely and uniformly distributed. In thiscase, when a steel sheet, in which residual dislocation and an interfacetogether with epsilon martensite are present, was affected by anexternal shock, a rate of conversion from energy to thermal energy wasincreased. Thus, damping capacity is improved, so damping properties maybe excellent.

According to the related art, when a room temperature SDC value is0.00015 or more, damping properties are excellent.

Through comprehensive results, when heat treatment is performedaccording to the present disclosure, an average particle diameter of 2μm or less is secured. Thus, it can be seen that excellent dampingproperties may be secured.

In the case of the Comparative Examples, except for a ComparativeExample heat treated at 700° C., the area fraction of epsilon martensiteis high, as compared to the Example. On the other hand, an averageparticle diameter of a microstructure is great. Thus, the dampingperformance is inferior.

While exemplary embodiments have been shown and described above, it willbe apparent to those skilled in the art that modifications andvariations could be made without departing from the scope of the presentinvention as defined by the appended claims.

1. A high Mn steel sheet having excellent damping properties, comprising0.1 wt % or less of carbon (C), 8 wt % to 30 wt % of manganese (Mn), 0.1wt % or less of phosphorus (P), 0.02 wt % or less of sulfur (S), 0.1 wt% or less of nitrogen (N), 0 wt % to 1.0 wt % of titanium (Ti), and iron(Fe) and inevitable impurities, wherein a microstructure containsepsilon martensite and austenite, and an average particle diameter ofeach of the martensite and the austenite is 2 μm or less.
 2. The high Mnsteel sheet having excellent damping properties of claim 1, wherein thesteel sheet additionally comprises one, two, or more among 0 wt % to 3wt % of silicon (Si), 0.005 wt % to 5.0 wt % of chromium (Cr), 0.005 wt% to 2.0 wt % of nickel (Ni), 0.005 wt % to 0.5 wt % of niobium (Nb),0.0001 wt % to 0.01 wt % of boron (B), 0.005 wt % to 0.5 wt % ofvanadium (V), and 0.005 wt % to 1 wt % of tungsten (W).
 3. The high Mnsteel sheet having excellent damping properties of claim 1, wherein aroom temperature specific damping capacity (SDC) value of the steelsheet is 0.00015 or more.
 4. A method for manufacturing a high Mn steelsheet having excellent damping properties, comprising: heating a high Mnsteel sheet, including 0 wt % to 0.1 wt % or less of carbon (C), 8 wt %to 30 wt % of manganese (Mn), 0.1 wt % or less of phosphorus (P), 0.02wt % or less of sulfur (S), 0.1 wt % or less of nitrogen (N), 0 wt % to1.0 wt % of titanium (Ti), andiron (Fe) and inevitable impurities to aheat treatment temperature of Ac1° C. to Ac3+50° C. at a heating rate of0.01° C./s to 200° C./s; maintaining the high Mn steel sheet for 0.01second to 24 hours at the heat treatment temperature; and cooling thehigh Mn steel sheet to room temperature at a cooling rate of 0.01° C./sor more.
 5. The method for manufacturing high Mn steel sheet havingexcellent damping properties of claim 4, wherein a microstructure of thehigh Mn steel sheet has epsilon martensite and austenite.
 6. The methodfor manufacturing high Mn steel sheet having excellent dampingproperties of claim 4, wherein the steel sheet additionally comprisesone, two, or more among 0 wt % to 3 wt % of silicon (Si), 0.005 wt % to5.0 wt % of chromium (Cr), 0.005 wt % to 2.0 wt % of nickel (Ni), 0.005wt % to 0.5 wt % of niobium (Nb), 0.0001 wt % to 0.01 wt % of boron (B),0.005 wt % to 0.5 wt % of vanadium (V), and 0.005 wt % to 1 wt % oftungsten (W).